In solid structures, in particular in load-bearing structures of, for example, bridges, buildings, galleries, railways, retaining walls, dams, dykes, slabs and beams of buildings, underground piping and structures of urban subways, and the like, it may be important to monitor, in several points, significant parameters, particularly mechanical stresses (and thus those forces and/or pressures causing the latter) to which the structure is subjected in those locations. In the present description, solid structures are considered such as structures made from construction material, for example cement, concrete, mortar, and the like.
Such monitoring, which is carried out either periodically or continuously, can be useful both in the step of initial construction and during the life of a solid structure.
To this purpose, electronic monitoring devices are known which use sensors being capable of offering a good performance while being cost-effective. Usually, these electronic devices are directly applied to the outer surface of the solid structure to be monitored, or within recesses that are previously provided therein and that are accessible from the outside.
To enhance the monitoring, in view of achieving a reliable evaluation of the solid structure, in terms of safety, aging, reaction to varying atmospheric conditions, and the like, approaches have been also developed wherein monitoring electronic devices are completely embedded, i.e. “buried” into the material (for example reinforced concrete) of which the solid structure to be monitored is made.
U.S. Pat. No. 6,950,767 describes an electronic monitoring device such as a system packaged in one container, which includes several parts that are assembled on a substrate, such as integrated circuits, sensors, antennas, capacitors, batteries, memories, control units and the like, which are implemented in various “chips” that are connected to each other. The approach described in U.S. Pat. No. 6,950,767 is a so-called approach of the “System in Package” type (SiP). It should be understood, however, that a SiP, which is intended to be first “drowned” in a construction material (e.g. liquid concrete, which is then intended to cure) and then remain “buried” within the solid structure, is subjected to critical conditions, for example due to the very high pressures thereon, which can even be as high as several hundreds of atmospheres. In addition, a number of other causes of wear exist, over time, for example, due to water infiltrations, which are capable of damaging the system. Accordingly, in the above-mentioned field of application, the approach described in U.S. Pat. No. 6,950,767 may not be fully satisfactory in terms of reliability.
Other prior art approaches use the piezo-resistive effect, i.e. the dependence between an electric signal generated by a piezo-resistive sensor and a mechanical stress (that is, strain, i.e., compression or tension) experienced by the material (for example, silicon) which the sensor has been manufactured from. The mechanical stress can be, in turn, representative of a pressure and/or force to which the sensor is subjected. The ratio between the force applied and the electric signal generated (sensitivity) depends on the reaction of the material (silicon) to the stresses, which, in turn, depends on the crystal orientation of the silicon. The sensitivity is thus a function of the direction in which the force, and the consequent mechanical stress, are applied.
The overall intensity as measured by the above-mentioned prior art devices comprises a contribution deriving from the vertical component of the force, but also a contribution deriving from the horizontal (or “lateral”, in an equivalent definition) component of the force. If it is desired to detect the vertical component of the force, and in case this component (weight force) is much greater than the lateral component, despite the intensity as detected by the sensors of the prior art devices is a good approximation of the result that is desired to be achieved, the above-mentioned prior art devices may have several drawbacks.
First of all, as stated above, the result of the sensor measurement does not correspond only to the vertical component. Furthermore, the result of the sensor measurement may not correspond even to the actual intensity of the force, as the two vertical and transversal components may result as being weighed differently by different sensitivity values. In addition, it is not possible to differentiate between the two components, nor measure them separately.
Furthermore, in case the lateral force is desired to be measured, this cannot be done simply by orientating the sensor so as to align the direction of maximum sensitivity with one of the crystalline axes. In fact, in this case, the result would be also influenced by the component along the crystalline axis that is aligned with the vertical, and thus is affected by the weight of the structure, which totally impairs the correctness and accuracy of the result.
Furthermore, generally, it should be understood that the building structures to be monitored can be anisotropic systems, wherein each point can be subjected to forces/stresses in different directions, i.e., having at least two components which are desired to be measured separately. Accordingly, with such a type of solid structure, the lateral mechanical flexural, compression or tension stresses being present in those points to be monitored, which are for example due to winds or particular structural configurations, also need to be known to obtain the most accurate monitoring for an effective maintenance of the structure.
From the above, it may be desired to improve the accuracy and correctness of detection, and particularly to allow individual detections of both the lateral and vertical components of the mechanical stress, or in other words, to detect the component of the force applied to a point in a specific (either vertical or lateral) direction of interest.